I think a little voice acting at specific points in a game can truly enhance it. Baldur's Gate II without David Warner just wouldn't be the same. Nevertheless, what I think Avellone is primarily referring to is the industry's over-reliance on voice acting, incorporating it far too often, making it (and cinematics) part of the gameplay itself and not merely a tool for its enhancement.

Not to mention the obvious difficulty in implementing changes to the game itself after recording has been completed.